An active noise cancellation (ANC) system in an earpiece-based audio device can be used to reduce background noise. The ANC system can form a compensation signal adapted to cancel background noise at a listening position inside the earpiece. The compensation signal is provided to an audio transducer (e.g., a loudspeaker) which generates an “anti-noise” acoustic wave. The anti-noise acoustic wave is intended to attenuate or eliminate the background noise at the listening position via destructive interference, so that only the desired audio remains. Consequently, the combination of the anti-noise acoustic wave and the background noise at the listening position results in cancellation of both and hence a reduction in noise.
ANC systems can generally be divided into feedforward ANC systems and feedback ANC systems. In a typical feedforward ANC system, a single feedforward microphone provides a reference signal based on the background noise captured at a reference position. The reference signal is then used by the ANC system to predict the background noise at the listening position so that it can be cancelled. Typically, this prediction utilizes a transfer function which models the acoustic path from the reference position to the listening position. The ANC is then performed to form a compensation signal adapted to cancel the noise, whereby the reference signal is inverted, weighted, and delayed or, more generally, filtered based on the transfer function.
Errors in a feedforward ANC can occur due to the difficulty in forming a transfer function which accurately models the acoustic path from the reference position to the listening position. Specifically, since the surrounding acoustic environment is rarely fixed, the background noise at the listening position is constantly changing. For example, the location and number of noise sources which form the resultant background noise can change over time. These changes affect the acoustic path from the reference position to the listening position. For example, a propagation delay of the background noise between the reference position and the listening position depends on the direction (or directions) the background noise is coming from. Similarly, the amplitude difference of the background noise at the reference position and at the listening position may depend on the direction.